The purpose of this investigation is to continue the study of the in vivo mechanisms of tumor regression following immunotherapy using hapten-modified tumor tissue. The knowledge obtained from these studies will be extended to develop a clinically relevant model for tumor therapy. Previously we demonstrated that L-phenylalanine mustard (PhM) is a hapten and that local injection of this hapten into the target tumor followed by an intravenous administration of syngeneic or allogeneic antiserum raised against PhM will cause complete regression of both allogeneic and syngeneic rat tumors. In experiments with allogeneic and syngeneic tumors a single treatment of one focus of tumor caused a systemic effect with regression of the untreated as well as treated tumors. Although the hapten, L-phenylalanine mustard, is cytotoxic treatment with this agent alone did not cause significant regression of the treated tumor. Treatment of appropriately modified tumors with anti PhM causes rapid development of massive haemorrhagic necrosis of tumor tissue, while the same treatment fails to produce this change in neutropenic rats. The most appropriate explanation for these observations is that PhM modification of tumors and treatment with anti PhM causes tumor damage through a vasculitis of the Arthus type. With this hypothesis in mind we wish to extned our analysis of the mechanisms accounting for tumor regression with PhM. Specifically, we plan to study the in vitro cytotoxicity of anti PhM antibody on PhM treated tumors; the role of polymorphonuclear leukocytes in tumor regression with PhM-anti PhM system; the in vivo role of complement in this system; the extent and sites of localiation of PhM, anti PhM, and complement in treated tumors; and explore the applications of non-oncolytic hapten (dinitraphenol) to tumor regression. The antibodies used in these various studies will be prepared by conventional methods as used by us in earlier studies but it is planned to produce and use monoclonal anti PhM antibodies as well.